Collaboration between wireless LAN access points using wired LAN infrastructure

ABSTRACT

A method for mobile communication includes arranging a plurality of access points in a wireless local area network (WLAN) to communicate on a common frequency channel with a mobile station, and linking the access points together by cables in a wired local area network (LAN). When one or more of the access points receive an uplink signal transmitted over the WLAN by the mobile station on the common frequency channel, the access points send messages over the LAN and arbitrate among themselves based on the messages so as to select one of the access points to respond to the uplink signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/272,686, filed Oct. 17, 2002, which claims the benefit of U.S.Provisional Patent Application No. 60/377,650, filed May 6, 2002, and isa continuation-in-part of U.S. patent application Ser. No. 10/214,271,filed Aug. 7, 2002, whose disclosure is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to local area network (LAN)communications, and specifically to methods and devices for improvingthe performance of wireless LANs.

BACKGROUND OF THE INVENTION

Wireless local area networks (WLANs) are gaining in popularity, and newwireless applications are being developed. The original WLAN standards,such as “Bluetooth” and IEEE 802.11, were designed to enablecommunications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEEworking groups have defined the 802.11a, 802.11b and 802.11g extensionsto the original standard, in order to enable higher data rates. The802.11a standard, for example, envisions data rates up to 54 Mbps overshort distances in a 5 GHz band, while 802.11b defines data rates up to22 Mbps in the 2.4 GHz band. In the context of the present patentapplication and in the claims, the term “802.11” is used to refercollectively to the original IEEE 802.11 standard and all its variantsand extensions, unless specifically noted otherwise.

The theoretical capability of new WLAN technologies to offer enormouscommunication bandwidth to mobile users is severely hampered by thepractical limitations of wireless communications. Indoor propagation ofradio frequencies is not isotropic, because radio waves are influencedby building layout and furnishings. Therefore, even when wireless accesspoints are carefully positioned throughout a building, some “blackholes” generally remain—areas with little or no radio reception.Furthermore, 802.11 wireless links can operate at full speed only underconditions of high signal/noise ratio. Signal strength scales inverselywith the distance of the mobile station from its access point, andtherefore so does communication speed. A single mobile station with poorreception due to distance or radio propagation problems can slow downWLAN access for all other users in its basic service set (BSS—the groupof mobile stations communicating with the same access point).

The natural response to these practical difficulties would be todistribute a greater number of access points within the area to beserved. If a receiver receives signals simultaneously from two sourcesof similar strength on the same frequency channel, however, it isgenerally unable to decipher either signal. The 802.11 standard providesa mechanism for collision avoidance based on clear channel assessment(CCA), which requires a station to refrain from transmitting when itsenses other transmissions on its frequency channel. In practice, thismechanism is of limited utility and can place a heavy burden ondifferent BSSs operating on the same frequency channel.

Therefore, in high data-rate 802.11 WLANs known in the art, accesspoints in mutual proximity must use different frequency channels.Theoretically, the 802.11b and 802.11g standards define 14 frequencychannels in the 2.4 GHz band, but because of bandwidth and regulatorylimitations, WLANs operating according to these standards in the UnitedStates actually have only three different frequency channels from whichto choose. (In other countries, such as Spain, France and Japan, onlyone channel is available.) As a result, in complex, indoor environments,it becomes practically impossible to distribute wireless access pointsclosely enough to give strong signals throughout the environment withoutsubstantial overlap in the coverage areas of different access pointsoperating on the same frequency channel.

Access points in a WLAN system are typically interconnected by a wiredLAN to communicate with a hub. The LAN serves as a distribution system(DS) for exchanging data between the access points and the hub. Thisarrangement enables the mobile stations to send and receive data throughthe access points to and from external networks, such as the Internet,via an access line connected to the hub.

Most commonly, the LAN used as a DS is an Ethernet LAN, operating inaccordance with the Carrier Sense Multiple Access with CollisionDetection (CSMA/CD) method of media access control (MAC) defined in IEEEStandard 802.3 (2000 Edition), which is incorporated herein byreference. The terms “Ethernet,” “CSMA/CD” and “802.3” are used in theart interchangeably to refer to LANs of this type. Ethernet LANs aretypically capable of carrying data at high speeds—greater than theaggregate speed of wireless communications between the access points andmobile stations. For example, a 100BASE-T Ethernet LAN is capable ofcarrying data over twisted pair cabling at 100 Mb/s. Message latency onthe LAN is relatively high, however, generally on the order ofmilliseconds, due mainly to collision avoidance mechanisms specified bythe 802.3 standard and the lack of a fragmentation mechanism at the802.3 MAC layer. Another factor contributing to latency in Ethernet LANsis that the minimum frame size permitted by the standard is 64 bytes(plus 8 more bytes for the frame preamble and start frame delimiter),while the maximum frame size is more than 1500 bytes.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to providemethods and devices for enhancing the coverage and speed of WLANsystems.

It is a further object of some aspects of the present invention toprovide methods and devices that enable a wired LAN to be used forhigh-speed, low-latency communications.

In preferred embodiments of the present invention, a WLAN systemcomprises multiple wireless access points distributed within a serviceregion. The access points are linked together by cables in a local areanetwork (LAN), typically an Ethernet LAN, which conveys data to and frommobile stations served by the access points. In order to providecomplete coverage of the service region, with strong communicationsignals throughout the region, the access points are preferably closelyspaced, and their areas of coverage may substantially overlap oneanother, unlike WLANs known in the art.

In order to deal with this overlap, the access points communicate amongthemselves using a novel, low-latency protocol over the LAN. When amobile station sends an uplink message attempting to initiatecommunications in a given frequency channel, a number of access pointsoperating in this frequency channel may typically receive the message.These access points arbitrate among themselves by sending messages overthe LAN, using the novel protocol to determine which access point willrespond to the mobile station. The arbitration must be completedpromptly, typically well below 10 μs. If the access points were limitedto communicating over the LAN using Ethernet protocols, they would beunable to complete the arbitration within this tight limit because ofthe high latency inherent in Ethernet, as described above. Therefore,each access point receiving the uplink message preempts its Ethernetcommunications immediately, and uses the novel protocol of the presentinvention instead to send and receive the messages necessary forarbitration. Standard Ethernet transmissions may resume afterwards.

The use of the arbitration mechanism of the present invention allowsaccess points to be deployed within the service region as closely asdesired while avoiding mutual interference. As a result, mobile stationseverywhere in the service region experience good radio coverage, without“black holes,” and can operate at optimal speed. Since the arbitrationmessaging among the access points takes advantage of an existing LANamong the access points (or a LAN that would be deployed as a DS for theWLAN in any case), the improved performance of the WLAN is achievedwithout substantial added hardware, by means of a very simpleinstallation procedure.

Although preferred embodiments described herein are directed primarilyto improving the coverage of WLAN systems, the principles of the presentinvention may be applied for other purposes, as well. Thus, the presentinvention may be employed to provide nodes in a LAN with dual MACcapabilities: a high-throughput MAC layer, such as a 100 Mb/s EthernetMAC layer, used for general data communications; and a separatelow-latency MAC layer, which is invoked when needed for sending short,high-priority messages, which are typically a microsecond or less induration. The high-speed MAC can be used, for example, forsynchronization and control signals that require low latency, andtherefore cannot be carried over Ethernet. Ordinarily, in the absence ofthe low-latency MAC, additional cabling among the nodes would berequired to carry these signals. The present invention resolves thisdeficiency of the prior art, allowing LAN cabling and equipment to beused for dual purposes.

There is therefore provided, in accordance with a preferred embodimentof the present invention, a method for mobile communication, including:

arranging a plurality of access points in a wireless local area network(WLAN) to communicate on a common frequency channel with a mobilestation;

linking the access points together by cables in a wired local areanetwork (LAN);

receiving at one or more of the access points an uplink signaltransmitted over the WLAN by the mobile station on the common frequencychannel;

sending one or more messages over the LAN among the access points,responsive to receiving the uplink signal;

arbitrating among the access points based on the messages so as toselect one of the access points to respond to the uplink signal; and

transmitting a response from the selected one of the access points tothe mobile station.

Preferably, linking the access points includes arranging the accesspoints to convey data to and from the mobile station via the LAN, inaddition to sending the messages over the LAN responsive to receivingthe uplink signal. Most preferably, arranging the access points toconvey the data includes configuring the access points to convey thedata in accordance with a first media access control (MAC) protocolcharacterized by a first latency, and sending the messages includesusing a second MAC protocol, having a second latency lower than thefirst latency, to send the messages responsive to receiving the uplinksignal. Typically, the first MAC protocol includes an Ethernet protocol.

Further preferably, sending the messages includes preempting conveyingthe data in accordance with the first MAC protocol in order to send themessages using the second MAC protocol. Typically, preempting conveyingthe data includes invoking a collision-avoidance mechanism provided bythe first MAC protocol. Most preferably, preempting conveying the dataincludes interrupting transmission of a frame of the data in accordancewith the first MAC protocol.

Preferably, sending the one or more messages includes sending broadcastmessages from the access points receiving the uplink signal to theplurality of the access points.

Further preferably, arbitrating among the access points includesreceiving and processing the messages at each of the plurality of theaccess points, so that each of the one or more of the access pointsreceiving the uplink signal determines which one of the access points isto be selected to respond to the uplink signal. Most preferably,processing the messages includes selecting, responsive to the messages,the one of the access points that was first to receive the uplinksignal.

Preferably, the access points have respective service areas, and whereinarranging the plurality of the access points includes arranging theaccess points so that the service areas substantially overlap.

In a preferred embodiment, arranging the plurality of the access pointsincludes arranging the access points to communicate with the mobilestation substantially in accordance with IEEE Standard 802.11, andarbitrating among the access points includes selecting the one of theaccess points to respond to the uplink signal within a time limitimposed by the IEEE Standard 802.11 for acknowledging the uplink signal.

There is also provided, in accordance with a preferred embodiment of thepresent invention, a method for network communication, including:

linking a plurality of nodes together in a local area network (LAN);

conveying data over the LAN among the nodes in accordance with a firstmedia access control (MAC) protocol characterized by a first latency;and

preempting conveying the data in accordance with the first MAC protocolin order to pass a message over the LAN among the nodes using a secondMAC protocol, having a second latency lower than the first latency.

In a preferred embodiment, the first MAC protocol includes an Ethernetprotocol, and preempting conveying the data includes asserting a signalin accordance with a media independent interface (MII) between physicaland MAC layers of the Ethernet protocol.

Additionally or alternatively, preempting conveying the data includesinvoking a collision-avoidance mechanism provided by the first MACprotocol.

Preferably, preempting conveying the data includes interruptingtransmission of a frame of the data in accordance with the first MACprotocol.

Most preferably, conveying the data includes establishing asynchronization between the nodes on the LAN in accordance with thefirst MAC protocol, and preempting conveying the data includes using theestablished synchronization to send the message using the second MACprotocol.

Additionally or alternatively, conveying the data includes sending dataframes including a first type of error detection code, and whereinpreempting conveying the data includes sending the message with a secondtype of error detection code, different from the first type.

There is additionally provided, in accordance with a preferredembodiment of the present invention, a system for mobile communication,including:

cables arranged to form a wired local area network (LAN); and

a plurality of access points interconnected by the LAN and arranged in awireless local area network (WLAN) to communicate on a common frequencychannel with a mobile station, the access points being adapted, uponreceiving at one or more of the access points an uplink signaltransmitted over the WLAN by the mobile station on the common frequencychannel, to send one or more messages over the LAN among the accesspoints, responsive to receiving the uplink signal, and to arbitrateamong the access points based on the messages so as to select one of theaccess points to respond to the uplink signal, and to transmit aresponse from the selected one of the access points to the mobilestation.

There is further provided, in accordance with a preferred embodiment ofthe present invention, a system for network communication, including:

cables arranged to form a wired local area network (LAN); and

a plurality of nodes, which are linked together by the LAN and areadapted to convey data over the LAN among the nodes in accordance with afirst media access control (MAC) protocol characterized by a firstlatency, and which are further adapted to preempt conveying the data inaccordance with the first MAC protocol in order to pass a message overthe LAN among the nodes using a second MAC protocol, having a secondlatency lower than the first latency.

There is moreover provided, in accordance with a preferred embodiment ofthe present invention, access point apparatus for deployment in awireless local area network (WLAN) as one of a plurality of accesspoints for mobile communication, the apparatus including:

a radio transceiver, which is configured to communicate on apredetermined frequency channel with a mobile station;

a physical layer interface, for connecting the access point to a wiredlocal area network (LAN) interconnecting the access points; and

processing circuitry, which is adapted, when the transceiver receives anuplink signal transmitted over the WLAN by the mobile station on thepredetermined frequency channel, to send and receive messages via thephysical layer interface over the LAN to and from the plurality ofaccess points, and to arbitrate among the access points based on themessages so as to select one of the access points to respond to theuplink signal, and to control the transceiver so that the transceiverreturns a response to the mobile station subject to the arbitrationprotocol.

Preferably, the processing circuitry is adapted to preempt conveying thedata in accordance with the first MAC protocol in order to send themessages using the second MAC protocol. Most preferably, the processingcircuitry includes a multiplexer, which is adapted invoke acollision-avoidance mechanism provided by the first MAC protocol inorder to preempt conveying the data. Further preferably, the processingcircuitry includes a MAC processor, which is adapted to generate framesof the data for transmission in accordance with the first MAC protocol,and the multiplexer is adapted to use the collision-avoidance mechanismin order to cause the MAC processor to interrupt transmission of one ofthe frames of the data.

There is furthermore provided, in accordance with a preferred embodimentof the present invention, node apparatus for deployment as one of aplurality of nodes in a local area network (LAN), the apparatusincluding:

a physical layer interface, for connecting the node to the LAN; and

processing circuitry, which is adapted to convey data via the physicallayer interface over the LAN in accordance with a first media accesscontrol (MAC) protocol characterized by a first latency, and which isfurther adapted to preempt conveying the data in accordance with thefirst MAC protocol in order to pass a message over the LAN using asecond MAC protocol, having a second latency lower than the firstlatency.

Preferably, the processing circuitry includes:

a first MAC processor, for conveying the data in accordance with thefirst MAC protocol;

a second MAC processor, for sending and receiving the message inaccordance with the second MAC protocol; and

a multiplexer, coupling the first and second MAC processors to thephysical layer interface, which is adapted to preempt the first MACprocessor so as to enable the second MAC processor to send the message.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a WLAN system,in accordance with a preferred embodiment of the present invention;

FIG. 2 is a block diagram that schematically shows details of accesspoints in a WLAN system, in accordance with a preferred embodiment ofthe present invention;

FIG. 3 is a block diagram that schematically illustrates a communicationprotocol stack with dual MAC layers, in accordance with a preferredembodiment of the present invention;

FIG. 4 is a block diagram that schematically illustrates a messagepacket exchanged between access points in a WLAN system, in accordancewith a preferred embodiment of the present invention; and

FIG. 5 is a flow chart that schematically illustrates a method forarbitrating among wireless access points in a WLAN system, in accordancewith a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a block diagram that schematically illustrates a wireless LAN(WLAN) system 20, in accordance with a preferred embodiment of thepresent invention. System 20 comprises multiple access points 22, whichare configured for data communication with mobile stations 24. Themobile stations typically comprise computing devices, such as desktop,portable or handheld devices, as shown in the figure. In the exemplaryembodiments described hereinbelow, it is assumed that the access pointsand mobile stations communicate with one another in accordance with oneof the standards in the IEEE 802.11 family and observe the 802.11 mediumaccess control (MAC) layer conventions. Details of the 802.11 MAC layerare described in ANSI/IEEE Standard 801.11 (1999 Edition), andspecifically in Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications, which is incorporated herein byreference. The principles of the present invention, however, are notlimited to the 802.11 standards, and may likewise be applied tosubstantially any type of WLAN, including HiperLAN, Bluetooth andhiswan-based systems.

Access points 22 are connected to an Ethernet switching hub 26 by awired LAN 28, which serves as the distribution system (DS) forexchanging data between the access points and the hub. As noted above,this arrangement enables mobile stations 24 to send and receive datathrough access points 22 to and from an external network 30, such as theInternet, via an access line 32 connected to hub 26. LAN 28 is typicallya 100BASE-TX LAN, operating in half-duplex mode, as specified by the802.3 standard. Alternatively, LAN 28 may comprise substantially anyEthernet standard LAN.

As described in the above-mentioned U.S. patent application(“Collaboration between Wireless LAN Access Points”), access points 22in system 20 are preferably closely spaced, so that radio waves maytypically reach mobile station 24 from multiple access pointssimultaneously on the same frequency channel. By the same token, radiomessages transmitted by mobile station 24 may be received at about thesame time by multiple access points. In WLAN systems known in the art,under these circumstances, mobile station 24 would receive downlinkmessages from two or more of the access points, which would probablyresult in inability of the mobile station to communicate with any of theaccess points. In preferred embodiments of the present invention, theaccess points collaborate to resolve this conflict by exchangingarbitration messages with one another using a novel, high-speed protocolover LAN 28, as described hereinbelow. Preferably, the arbitrationmessages are broadcast by all the access points that receive an uplinksignal from a given mobile station, to all the other access points.Based on the arbitration messages, the access points decide which accesspoint is to serve a given mobile station (usually the closest accesspoint to the mobile station, meaning the first access point to send outan arbitration message over LAN 28 in response to a given uplinkmessage). The other access points meanwhile refrain from interfering.

Ordinarily, in a conventional WLAN, when an access point receives anuplink message from a mobile station, it answers immediately with anacknowledgment (ACK). If the mobile station does not receive the ACKwithin a given timeout period (known as the interframe space, or IFS),typically 10 μs, it subsequently submits an automatic repeat request(ARQ). Ultimately, the mobile station will treat the message exchange ashaving failed if it repeatedly does not receive the required ACK.Therefore, to maintain 802.11 compatibility in system 20, one—and onlyone—of the receiving access points must return an ACK to mobile station24 within the 10 μs limit. This constraint requires that the arbitrationprocess among the access points be completed in substantially less than10 μs. For this purpose, access points 22 are provided with dual MACfunctions: an Ethernet MAC for conventional data communications, and anovel low-latency MAC for arbitration, as described below.

FIG. 2 is a block diagram that schematically shows details of accesspoints 22, in accordance with a preferred embodiment of the presentinvention. Access point 22 is connected to hub 26 by wires 33 of LAN 28.Hub 26 typically comprises a standard Ethernet switching hub, as isknown in the art, which is additionally programmed to recognize andrapidly switch the (non-Ethernet) arbitration messages exchanged amongthe access points. Access point 22 comprises a physical layer interface(PHY) 34, which transmits and receives signals over wires 33 inaccordance with the 100BASE-TX PHY layer specification of the 802.3standard. Preferably, PHY 34 operates in a half-duplex mode, as providedby the standard.

A multiplexer 35 attaches PHY 34 to two different MAC processors: anEthernet frame processor 36 and a collaboration message processor 38. Asa rule, the multiplexer gives priority to delivering outgoingarbitration messages from the message processor, blocking the frameprocessor and preempting any transmission of pending Ethernet frames inthe meanwhile. Based on these arbitration messages, processor 38interacts with and controls a WLAN transceiver 37. Transceivers 37communicate over the air with mobile stations 24 in accordance with theapplicable WLAN standard.

The elements of access point 22 shown in FIG. 2 may comprise individual,separate components, or they may alternatively be combined together in asingle integrated circuit chip or chip set. Although multiplexer 35 andmessage processor 38 are novel and unique to the present invention, theother elements of the access point shown in FIG. 2 (including PHY 34,Ethernet frame processor 36 and transceiver 37) are available off-shelfas standard components. The multiplexer, Ethernet frame processor andmessage processor may also be implemented as software processes runningon a single microprocessor, as long as the processing speed of themicroprocessor is sufficient.

FIG. 3 is a block diagram that schematically illustrates a protocolstack implemented by the components of access point 22, in accordancewith a preferred embodiment of the present invention. PHY 34 implementsa standard physical layer protocol 42, in accordance with the 802.3standard, such as the 100BASE-TX protocol. The functions of a MACprotocol layer 44, however, are divided among several components.Ethernet frame processor 36 implements a standard 802.3 MAC protocol 46.Message processor 38, on the other hand, uses a novel low-latency MACprotocol 48 for arbitration messaging among the access points. Acombination (COMBO) layer 50 is provided by multiplexer 35 to interfacebetween the physical layer protocol and the alternative MAC protocols.

Preferably, COMBO layer 50 uses a Machine-Independent Interface (MII) tointerface with the physical and Ethernet MAC layers, and optionally withlow-latency MAC 48, as well. The MII, as defined in detail in Chapter 22of the 802.3 standard, provides standard primitives for communicationbetween the Ethernet MAC layer and the 100BASE-TX PHY layer. By usingthese primitives in the manner provided by the 802.3 standard, theoperation of COMBO layer 50 is transparent to the Ethernet MAC and PHYlayers. In other words, these layers operate in the conventionalfashion, and need not be modified to accommodate low-latency MAC 48.

At a higher protocol level, network and application layers 52 areresponsible for conveying data to and from mobile stations 24 betweenWLAN transceiver 37 and LAN 28. These conventional functions are beyondthe scope of the present invention, and their implementation will beapparent to those skilled in the art.

An access point collaboration layer 54 is responsible for generatingarbitration messages to be transmitted over LAN 28 via high-speed MAClayer 48 and for receiving and processing incoming arbitration messagesfrom other access points. Layer 54 uses the arbitration messageinformation to determine which of the access points should respond to agiven uplink message received by transceiver 37 and outputs controlsignals to the transceiver accordingly. These operations, and associateddetails of the operation of low-latency MAC layer 48 and COMBO layer 50,are described further hereinbelow with reference to FIG. 5.

FIG. 4 is a block diagram that schematically illustrates a broadcastpacket 60 sent over LAN 28 by one of access points 22, in accordancewith a preferred embodiment of the present invention. Packet 60 is usedby the access points to convey arbitration messages to the other accesspoints upon receiving uplink communications from one of mobile stations24, as described below with reference to FIG. 5. The packet comprises asource address (SA) 62, a message body 64 and an error checking code 66,typically a cyclic redundancy code (CRC), as is known in the art.

For rapid communications between the access points, it is desirable thatpacket 60 be as short as possible, most preferably no more than 16 bits.Thus, SA 62 simply identifies the sending access point, in a unique,proprietary format, which also allows hub 26 to recognize the packet asa broadcast packet. Since the hub distributes the packet to all theaccess points, there is no need for a destination address. Hub 26 notonly has the capabilities of a standard Ethernet switching hub, but alsohas added hardware and software capabilities that enable it to recognizepacket 60 and distribute it with highest priority. For this purpose, hub26 preferably includes dedicated broadcast circuitry, since otherwisethe standard 802.3 switching circuits would regard packet 60 aserroneous and would therefore drop it. Most preferably, hub 26, likeaccess point 22, has an added a buffer layer between the standard PHYlayer and two different MAC layers: the standard 802.3 switching MAC andthe novel low-latency broadcast MAC of the present invention. Since100BASE-TX uses synchronous links (“always on”), hub 26 preferablyincludes an elastic buffer (not shown) for use in broadcasting packet 60from one input port to many output ports in parallel.

Message body 64 identifies the mobile station that sent the uplinkmessage reported by packet 60. For efficient communications, the mobilestation identification is abridged, by hashing to a 16-bit code, forexample. Message processor 38 in each of the access points receivingpacket 60 decodes SA 62 and message body 64. The message processor thusresolves the identities of both the mobile station that sent the uplinkmessage and the access point that received the uplink message and issuedpacket 60. Based on the contents of packets that it receives and thetimes at which it receives them, the message processor decides whetherthis access point should respond to the uplink message. Typically, thefirst access point to send out a broadcast packet in response to a givenuplink message is chosen to respond to the message. Optionally, messagebody 64 may include other parameters, such as the power level of thereceived uplink message and/or an identification of the antenna on whichthe access point received the message. (For diversity purposes, accesspoints generally have multiple antennas.) These additional parametersmay be used, in addition to or instead of the time of receipt of packet60, in arbitrating among the access points.

Code 66 is preferably an 8- or 16-bit CRC, which is used by messageprocessor 38 to verify the correctness of the contents of packet 60.Most preferably, code 66 uses a different coding scheme from thatprovided by the 802.3 standard. As a result, if packet 60 isaccidentally passed to Ethernet MAC processor 36, the Ethernet MAC layerwill be unable to correctly decode the CRC and will therefore discardthe packet.

FIG. 5 is a flow chart that schematically illustrates a method forestablishing communications between mobile station 24 and one of accesspoints 22 in system 20, in accordance with a preferred embodiment of thepresent invention. Further details of this method are described in theabove-mentioned U.S. Patent Application (which uses a dedicated, sharedmedium to exchange arbitration messages between the access points,rather than LAN 28). Access points 22 transmit beacon signals on theircommon frequency channel, giving the time base with which the mobilestation should synchronize its communications and indicating the BSSidentification (BSSID) of the access point. In 802.11 WLAN systems knownin the art, each access point has its own unique BSSID. In system 20,however, multiple access points share the same BSSID, so that theyappear logically to the mobile station to be a single, extended,distributed access point, which has multiple antennas at differentlocations. The time bases of the access points are mutually synchronizedusing synchronization messages sent over LAN 28 (in the form of packet60), and the beacon signals transmitted by the access points areinterlaced to avoid collision between them.

When mobile station 24 receives a beacon signal of sufficient strength,it extracts the BSSID and time base from the signal, and uses them tosend an uplink message, which is received by one or more of the accesspoints, at an uplink step 70. The actions of the mobile station in thisand other steps are completely in accordance with the 802.11 standard.In other words, the present invention can be implemented in a mannerthat is by definition transparent to and requires no modification ofexisting mobile stations. Typically, the first uplink signal sent by themobile station is an association request message that is addressed tothe BSSID and indicates the MAC address of the mobile station. Followingthis uplink message, one—and no more than one—of the receiving accesspoints must return an ACK to mobile station 24 within the 10 μs IFSlimit, as explained above.

To determine which of the access points will respond to the associationrequest message, access points 22 carry out an arbitration procedureusing LAN 28. For this purpose, message processors 38 in all accesspoints that received the uplink message from mobile station 24 preparebroadcast packets 60, at a packet generation step 72, in order to givenotice to the other access points that they have received an uplinkmessage. High-speed MAC layer 48 notifies COMBO layer that it has apacket ready to transmit, preferably by setting a transmit enable flag.For example, assuming the high-speed MAC and COMBO layers communicate inaccordance with the MII defined by the 802.3 standard, the high-speedMAC layer asserts the TX_EN signal synchronously with the first nibbleof the transmitted packet. It continues to assert this flag until theentire packet has been transmitted.

As soon as low-latency MAC layer 48 of message processor 38 notifiesCOMBO layer 50 of multiplexer 35 that it has a packet to transmit, theCOMBO layer immediately breaks off any Ethernet communications by accesspoint 22, at an Ethernet blocking step 74. Preferably, the COMBO layernotifies Ethernet MAC layer 46 that a collision has been detected on theLAN, by asserting the COL signal provided by the MII of the 802.3standard. When such a collision condition occurs, the Ethernet MAC layerterminates transmission of any frames in progress, and defers furthertransmissions as long as the COL flag remains asserted. If the COMBOlayer was in the process of transmitting an Ethernet frame, itimmediately stops transmission and requests that PHY layer 42deliberately corrupt the contents of the frame in such a manner that areceiver will detect the corruption with the highest degree ofprobability. For this purpose, the COMBO layer preferably asserts theTX_ER and TX_EN signals, as provided by the MII, on its interface withthe PHY layer. In response, as specified in section 22.2.2.8 of the802.3 standard, the PHY layer will emit one or more symbols that are notpart of the valid data or delimiter set provided by the standard. Thesesymbols will cause all receivers of the frames to immediately discardthem.

After asserting the COL and TX_ER flags (preferably for no more than oneclock period), COMBO layer 50 transmits the broadcast message preparedby low-latency MAC layer 48, at a broadcast transmission step 76. TheCOMBO layer preferably asserts the TX_EN flag in order to instruct PHYlayer 42 to transmit the packet. Even when the PHY layer is idle, itcontinues to transmit and receive idle symbols over LAN 28 in order tomaintain synchronization, as mandated by the 802.3 standard. Therefore,there is essentially no synchronization delay involved in beginning tosend or receive an arbitration broadcast packet over the LAN.

All access points 22 receive the broadcast packets sent over LAN 28, ata message reception step 78. When COMBO layer 50 receives one of thebroadcast packets, it passes the packet immediately to low-latency MAClayer 48. The MAC layer passes the message information to collaborationlayer 54, which arbitrates among the access points that sent broadcastpackets, at an arbitration step 80, in order to determine which accesspoint will respond to the uplink message received at step 70. The samearbitration takes place at all the access points. Each access point isable to determine whether it was first to send its message, or whetheranother access point preceding it, by comparing the time of receipt ofthese broadcast messages to the time at which the access point sent itsown broadcast message. (Access points operating on other frequencychannels, as well as access points on the same frequency channel thatdid not receive an uplink signal from the mobile station identified inthe broadcast message, may ignore the message.)

Typically, the access point that was able to send its broadcast messagefirst in response to an uplink message from a given mobile station is ina good position to continue communications with the mobile station.Therefore, all the access points independently choose this first accesspoint to respond to mobile station 24. The 802.11 standard supports alarge range of data rates for transmission (1 to 54 Mb/s). The mobilestation tries to transmit packets as fast as possible, link permitting.Therefore, in general, only the access points that are close enough tothe mobile station to receive the high-rate transmission will be incontention to serve the mobile station, and the winning access pointmust implicitly be among the best receivers of the uplink message inquestion.

Alternatively or additionally, other criteria, such as received signalpower, may be applied in choosing the “winning” access point, as long asthe criteria are applied uniformly by all the access points. Preferably,if a deadlock occurs (such as when two access points send theirbroadcast messages at the same instant), a predetermined formula, whichmay be based on the received signal power, is applied by all the accesspoints to resolve the deadlock uniformly.

The winning access point sends the required ACK message to mobilestation 24, at an acknowledgment step 82. As noted above, the ACK mustbe sent within a short time, typically 10 μs, and steps 70-80 must allbe completed within this time. Access points 22 are able to meet thistime constraint by using LAN 28 in the manner described above. Aftersending the ACK, the winning access point typically sends an associationresponse message to mobile station 24, and then continues its downlinktransmission to the mobile station as appropriate.

The winning access point continues serving the mobile station until themobile station sends another uplink message. The arbitration protocoldescribed above is then repeated. A different access point may be chosento serve the mobile station in the next round, particularly if themobile station has moved in the interim. Even if the mobile station hasmoved, there is no need to repeat the association protocol. As notedabove, all the access points belong to the same BSS, as though they werea single extended access point. Therefore, the same association of themobile station is therefore maintained even if the arbitration processamong the access points chooses a different “winner” to respond to thenext uplink packet from the mobile station.

The LAN communication architecture shown in FIG. 2 and the protocolstack shown in FIG. 3 are useful not only in improving the coverage ofWLAN systems, as described above, but also in other networkcommunication contexts. As noted above, the present invention may thusbe employed to provide nodes in a LAN with dual MAC capabilities: amedium-latency MAC layer, such as an Ethernet MAC layer, used forgeneral data communications; and a separate low-latency MAC layer, whichis invoked when needed for sending short, urgent messages. Low-latencyMAC layer 48 can be used, for example, for synchronization and controlsignals that require low latency, and therefore cannot be accommodatedby Ethernet MAC layer 46. The high-speed MAC and COMBO layers of thepresent invention can similarly be used in a dual-MAC configurationalongside other types of MAC and data link protocol layers known in theart. For example, the low-latency MAC layer could be used in a real-timelocation system, to use multiple radio propagation measurements tolocate people in an office building or plant.

It will thus be appreciated that the preferred embodiments describedabove are cited by way of example, and that the present invention is notlimited to what has been particularly shown and described hereinabove.Rather, the scope of the present invention includes both combinationsand subcombinations of the various features described hereinabove, aswell as variations and modifications thereof which would occur topersons skilled in the art upon reading the foregoing description andwhich are not disclosed in the prior art.

1. A method for mobile communication, comprising: arranging a pluralityof access points in a wireless local area network (WLAN) to communicateon a common frequency channel with a mobile station; linking the accesspoints together by cables in a wired local area network (LAN) so as toconvey data to and from the mobile station via the LAN in accordancewith a first media access control (MAC) protocol characterized by afirst latency; receiving at one or more of the access points an uplinksignal transmitted over the WLAN by the mobile station on the commonfrequency channel; sending one or more messages over the LAN among theaccess points, responsive to receiving the uplink signal, using a secondMAC protocol, having a second latency lower than the first latency;arbitrating among the access points based on the messages so as toselect one of the access points to respond to the uplink signal; andtransmitting a response from the selected one of the access points tothe mobile station.
 2. A method according to claim 1, wherein the firstMAC protocol comprises an Ethernet protocol.
 3. A method according toclaim 1, wherein sending the messages comprises preempting conveying thedata in accordance with the first MAC protocol in order to send themessages using the second MAC protocol.
 4. A method according to claim3, wherein preempting conveying the data comprises invoking acollision-avoidance mechanism provided by the first MAC protocol.
 5. Amethod according to claim 3, wherein preempting conveying the datacomprises interrupting transmission of a frame of the data in accordancewith the first MAC protocol.
 6. A method according to claim 1, whereinsending the one or more messages comprises sending broadcast messagesfrom the access points receiving the uplink signal to the plurality ofthe access points.
 7. A method according to claim 1, wherein arbitratingamong the access points comprises receiving and processing the messagesat each of the plurality of the access points, so that each of the oneor more of the access points receiving the uplink signal determineswhich one of the access points is to be selected to respond to theuplink signal.
 8. A method according to claim 7, wherein processing themessages comprises selecting, responsive to the messages, the one of theaccess points that was first to receive the uplink signal.
 9. A methodaccording to claim 1, wherein the access points have respective serviceareas, and wherein arranging the plurality of the access pointscomprises arranging the access points so that the service areassubstantially overlap.
 10. A method according to claim 1, whereinarranging the plurality of the access points comprises arranging theaccess points to communicate with the mobile station substantially inaccordance with IEEE Standard 802.11.
 11. A method according to claim10, wherein arbitrating among the access points comprises selecting theone of the access points to respond to the uplink signal within a timelimit imposed by the IEEE Standard 802.11 for acknowledging the uplinksignal.
 12. A method for network communication, comprising: linking aplurality of nodes together in a local area network (LAN); conveyingdata over the LAN among the nodes in accordance with a first mediaaccess control (MAC) protocol characterized by a first latency; andpreempting conveying the data in accordance with the first MAC protocolin order to pass a message over the LAN among the nodes using a secondMAC protocol, having a second latency lower than the first latency. 13.A method according to claim 12, wherein the first MAC protocol comprisesan Ethernet protocol.
 14. A method according to claim 12, whereinpreempting conveying the data comprises invoking a collision-avoidancemechanism provided by the first MAC protocol.
 15. A method according toclaim 12, wherein preempting conveying the data comprises interruptingtransmission of a frame of the data in accordance with the first MACprotocol.
 16. A system for mobile communication, comprising: cablesarranged to form a wired local area network (LAN); and a plurality ofaccess points interconnected by the LAN and arranged in a wireless localarea network (WLAN) to communicate on a common frequency channel with amobile station and to convey data to and from the mobile station via theLAN in accordance with a first media access control (MAC) protocolcharacterized by a first latency, the access points being adapted, uponreceiving at one or more of the access points an uplink signaltransmitted over the WLAN by the mobile station on the common frequencychannel, to send one or more messages over the LAN among the accesspoints, responsive to receiving the uplink signal, using a second MACprotocol, having a second latency lower than the first latency, and toarbitrate among the access points based on the messages so as to selectone of the access points to respond to the uplink signal, and totransmit a response from the selected one of the access points to themobile station.
 17. A system according to claim 16, wherein the firstMAC protocol comprises an Ethernet protocol.
 18. A system according toclaim 16, wherein the access points are adapted to preempt conveying thedata in accordance with the first MAC protocol in order to send themessages using the second MAC protocol.
 19. A system according to claim18, wherein the access points are arranged to preempt conveying the databy invoking a collision-avoidance mechanism provided by the first MACprotocol.
 20. A system according to claim 18, wherein the access pointsare adapted to preempt conveying the data by interrupting transmissionof a frame of the data in accordance with the first MAC protocol.
 21. Asystem according to claim 16, wherein the one or more messages comprisebroadcast messages, sent from the access points receiving the uplinksignal to the plurality of the access points.
 22. A system according toclaim 16, wherein the access points are adapted to receive and processthe messages so that each of the one or more of the access pointsreceiving the uplink signal determines which one of the access points isto be selected to respond to the uplink signal.
 23. A system accordingto claim 22, wherein the access points are adapted to select, responsiveto the messages, the one of the access points that was first to receivethe uplink signal.
 24. A system according to claim 16, wherein theaccess points have respective service areas, and wherein the accesspoints are arranged so that the service areas substantially overlap. 25.A system according to claim 16, wherein the access points are adapted tocommunicate with the mobile station substantially in accordance withIEEE Standard 802.11.
 26. A system according to claim 25, wherein theaccess points are adapted to select the one of the access points torespond to the uplink signal within a time limit imposed by the IEEEStandard 802.11 for acknowledging the uplink signal.
 27. A system fornetwork communication, comprising: cables arranged to form a wired localarea network (LAN); and a plurality of nodes, which are linked togetherby the LAN and are adapted to convey data over the LAN among the nodesin accordance with a first media access control (MAC) protocolcharacterized by a first latency, and which are further adapted topreempt conveying the data in accordance with the first MAC protocol inorder to pass a message over the LAN among the nodes using a second MACprotocol, having a second latency lower than the first latency.
 28. Asystem according to claim 27, wherein the first MAC protocol comprisesan Ethernet protocol.
 29. A system according to claim 27, wherein thenodes are adapted to preempt conveying the data by invoking acollision-avoidance mechanism provided by the first MAC protocol.
 30. Asystem according to claim 27, wherein the nodes are adapted to preemptconveying the data by interrupting transmission of a frame of the datain accordance with the first MAC protocol.
 31. Access point apparatusfor deployment in a wireless local area network (WLAN) as one of aplurality of access points for mobile communication, the apparatuscomprising: a radio transceiver, which is configured to communicate on apredetermined frequency channel with a mobile station; a physical layerinterface, for connecting the access point to a wired local area network(LAN) interconnecting the access points; and processing circuitry, whichis adapted to convey data to and from the mobile station over the LANvia the physical layer interface in accordance with a first media accesscontrol (MAC) protocol characterized by a first latency, and which isfurther adapted, when the transceiver receives an uplink signaltransmitted over the WLAN by the mobile station on the predeterminedfrequency channel, to send and receive messages via the physical layerinterface over the LAN to and from the plurality of access points usinga second MAC protocol, having a second latency lower than the firstlatency, and to arbitrate among the access points based on the messagesso as to select one of the access points to respond to the uplinksignal, and to control the transceiver so that the transceiver returns aresponse to the mobile station subject to the arbitration protocol. 32.Apparatus according to claim 31, wherein the first MAC protocolcomprises an Ethernet protocol.
 33. Apparatus according to claim 31,wherein the processing circuitry is adapted to preempt conveying thedata in accordance with the first MAC protocol in order to send themessages using the second MAC protocol.
 34. Apparatus according to claim31, wherein the messages sent and received by the processing circuitryresponsive to receiving the uplink signal comprise broadcast messages,sent from the access points receiving the uplink signal to the pluralityof the access points.
 35. Apparatus according to claim 31, wherein theprocessing circuitry is adapted to receive and process the messages sothat each of the one or more of the access points receiving the uplinksignal determines which one of the access points is to be selected torespond to the uplink signal.
 36. Apparatus according to claim 35,wherein the processing circuitry is adapted to select, responsive to themessages, the one of the access points that was first to receive theuplink signal.
 37. Apparatus according to claim 31, wherein thetransceiver is adapted to communicate with the mobile stationsubstantially in accordance with IEEE Standard 802.11.
 38. Apparatusaccording to claim 37, wherein the processing circuitry is adapted toselect the one of the access points to respond to the uplink signalwithin a time limit imposed by the IEEE Standard 802.11 foracknowledging the uplink signal.
 39. Node apparatus for deployment asone of a plurality of nodes in a local area network (LAN), the apparatuscomprising: a physical layer interface, for connecting the node to theLAN; and processing circuitry, which is adapted to convey data via thephysical layer interface over the LAN in accordance with a first mediaaccess control (MAC) protocol characterized by a first latency, andwhich is further adapted to preempt conveying the data in accordancewith the first MAC protocol in order to pass a message over the LANusing a second MAC protocol, having a second latency lower than thefirst latency.
 40. Apparatus according to claim 39, wherein the firstMAC protocol comprises an Ethernet protocol.